1. Field of the Invention
This invention relates to a crack shear displacement gage for use in property evaluation testing, quality control and the like of composite laminates.
2. Prior Art Statement
According to one known method for testing the toughness of, for example, a composite laminate, an end-notched flexure specimen of the material is supported at opposite ends in the manner of a beam, a concentrated vertical load is applied thereto, and the relationship between the amount of load and the load line displacement are recorded in the form of a chart. This enables the fracture toughness to be determined from the maximum load.
In the case of a brittle material, the maximum load is substantially the same as the critical load, namely the load at the time that crack growth begins, and can thus be used as an index of the fracture point. In the case of a tough composite laminate, however, it is the critical load that serves as the index of the fracture point. (See FIG. 8.) It is thus very important to be able to detect the critical load with high accuracy.
In the aforesaid test method employing an end-notched flexure specimen, however, it is not possible to determine the critical load from the chart showing the relationship between the load and the load line displacement.
The critical load can, however, be determined with high precision by measuring the "crack shear displacement." In this specification, the term "crack shear displacement (CSD)" is defined to mean the lateral shift occurring at an end of a test specimen between the portions above and below a lateral crack when a test specimen is provided with a lateral crack inwardly from an end face thereof and is then subject to bending.
The conventional displacement gage measures the amount of crack opening in the direction perpendicular to the crack face, making it impossible to determine the critical load from the measured value.